1. Field of the Invention
The present invention relates to an improvement of an ozone generator.
2. Description of the Prior Art
As is well known, ozone is provided with high oxidative activity so that it is employed as an oxidizing agent in a chemical industry since a long time ago. In addition to the above, ozone is widely employed also in sterilization, deodorizing, decoloring and the like uses, and in recent years particularly in fine chemical industry use.
In order to produce a large amount of ozone for industry use, there have been proposed many processes for producing ozone such as a process in which oxygen is irradiated with ultraviolet energy, a process in which the electrolysis of water is conducted and the like process. Among these processes, most widely employed is normally a process in which the silent electrical discharge is conducted between a pair of electrodes which is spaced apart from each other to provide an air gap therebetween, in which process dielectric is attached to at least one of the opposite electrodes and AC voltage is imposed across them to conduct the silent discharge therebetween through the dielectric so that a dry air or oxygen gas passed through the air gap is ozonized to produce ozone.
This process is disadvantageous in that the electrical discharge produces a large amount of heat wwhich increase the temperature of the air of oxygen gas passing through the air gap and in turn the temperature of both the electrodes and the dielectric contacting such air or oxygen gas. On the other hand, ozone is immediately decomposed to oxygen when its temperature increases, so that is is necessary to cool ozone in order to efficiently produce the same. As a result, in practice, a conventional ozone generator is provided with a construction shown in FIGS. 5 and 6, which conventional generator will be hereinbelow described. Such conventional ozone generator employs: a stainless steel cylinder 6 as an electrode; and a glass cylinder 8 concentrically disposed in the stainless steel cylinder 6 and acting as a dielectric, an inner surface of which glass cylinder 8 is provided with a conducting film 10 formed by a suitable process such as a vapor deposition of metal so as to provide the other electrode. Incidentally, in FIG. 5, the reference numeral 5 denotes a metallic casing in which the stainless steel cylinder 6 is mounted through partition boards 7a and 7b hermetically by welding. Inside the stainless steel cylinder 6, there is concentrically mounted through a spacer 9 the glass cylinder 8 which is closed at its one end. An air gap 3 is formed between the glass cylinder 8 and the stainless steel cyliner 6 so that the glass cylinder 8 is normally spaced apart from the stainless steel cylinder 6 by a distance of from 1 to 3 mm. The conducting film 10 is formed on the inner surface of the glass cylinder 8 through a painting process or a vapor deposition process, with which film 10 is brought into contact a brush-like contact element 11 connected with a conductor wire 12 which extends outward through an insulating tube 13 mounted on an end plate 5a of the casing 5 so as to be connected with an electric source unit 14.
On the other hand, outside the stainless steel cyliner 6, a cooling water chamber 15 is defined by the partition boards 7a and 7b, while a chamber 16 for the air or oxygen gas (hereinafter referred to as a raw gas) i.e., a raw gas chamber 16 and a chamber 17 for a mixture gas of ozone and the raw gas, i.e., a mixture gas chamber 17 are formed in an upper and lower portions of the casing 5, respectively. The cooling water chamer 15 is provided with an inlet aperture 15a and outlet aperture 15b, while the raw gas chamber 16 is provided with an inlet aperture 16a and the mixture gas chamber 17 is provided with an outlet aperture 17a, respectively.
Incidentally, as shown in FIG. 7, the spacer 9 is constructed of a stainless steel coil spring shaped into a ring-like configuration so as to not prevent the raw gas and the mixture gas of the same and ozone from freely passing through the air gap 3. The casing 5 is grounded.
When high voltage AC is imposed between the thus constructed casing 5 and the conducting film 10 by the use of the electric source unit 14, the electrical charge is accumulated in the glass cylinder 8 serving as dielectric to a predetermined voltage level at which the breakdown of insulation in the air gap occurs. When the thus accumulated electrical charge reaches such predetermined voltage level, thin columns of electrical discharges occur continuously, which columns vary in number according to the applied voltage and frequency. When the raw gas is fed from the inlet 16a through the raw gas chamber 16 to the air gap 3 subjected to the electrical discharge, a part of oxygen molecules contained in the raw gas is hit with electron to produce activated oxygens through which ozone is then produced.
In this case, generally, only about 10% of the supplied energy of the electrical discharge is used to produce ozone, and the remainder thereof is transformed into heat so that a large amount of heat is produced by such electrical discharge, which heat increases the temperature of the air gap 3. When the temperature of the air gap 3 is increased, the produced ozone is immediatley decomposed into oxygen so that in order to prevent such immediate decomposition of the produced ozone the cooling water is supplied to the cooling water chamber 15 to remove the heat produced by the electrical discharge.
The above description is made as to one set of the ozone generator. Also conventionally employed as a large-capacity ozone generator is one constructed of a plurality of such sets combined in parallel with each other.
The conventional ozone generator having the above construction is effective in the conventional use. However, inr ecent years, the need for high-purity ozone increases in fine chemical use which can not be satisfied with the ordinary-grade ozone such as one produced by the conventional ozone generator, so that the need for improvement of the conventional ozone generator increases. Hereinbelow, the reason for such need will be more specifically described. In the integrated circuit manufacturing plant (hereinafter referred to as the IC plant), in case that ozone is employed as the oxidizing agent, when trace amounts of impurities are contained in the ozonized oxygen, such impurities that is fine particles adhere to the integrated circuit to damage its ultrafine circuit network. Consequently, the IC plant is provided with a so-called clean room to prevent such damage from occurring. In the above-mentioned conventional ozone generator, when electrons hits the surfaces of the stainless steel cylinder 6 and the spacer 9 in the electrical discharge process, fine particles of stainless steel are emitted from such surfaces and are intermingled with the ozonized oxygen, so that such fine particles adhere to the integrated circuit. This is a disadvantage inherent in the conventional ozone generator.
On the other hand, there have been proposed some improvements in which: in place of the stainless steel cylinder, a metallic cylinder lined in its inner surface with suitable dielectric such as glass, ceramics and the like is employed so as to prevent electrons from directly hitting the metal. However, such proposal is disadvantageous in that: since metal and dielectric are different in thermal expansion coefficient from each other, a crack and blister are produced in the dielectric to produce electrical short circuit which produces high current passing through such crack and blister to cause breakage of the dielectric and the metallic electrode; and, in the above-mentioned construction, since the dielectric lining the metal does not adhere to the same to provide a void therebetween, wasteful electrical discharge is conducted in such void so that a part of the supplied electrical power is lost without any contribution to the production of ozone. As a result, the above conventional proposal is poor in commerical use.
In addition to the above, hitherto, further another improvement has been proposed, in which: in place of the stainless steel cylinder, a glass cylinder is employed; and, by using the electrical conductivity of the cooling water received in the cooling water chamber, high voltage AC is imposed between the cooling water and the conducting film provided in the inner surface of the glass cylinder. However, in recent years, in the IC plant, high-purity water being poor in electrical conductivity is generally employed so that the above conventional another improvement is not effective in practical use.